In recent years the problem of groundwater contamination by organic solvents and other petroleum products has reached significant proportions. The contaminating compounds involve organic solvents used, for example, in the electronics and chemical industries, and petroleum hydrocarbons. Many of these compounds are generally hydrophobic with low water solubilities resulting in the presence of separate underground liquid phases. Methods for cleaning up contaminated sites heretofore used consist of a combination of (a) removal of the contaminated soil; (b) withdrawal of liquids existing as a pool floating in the aquifer (a water-bearing stratum) or at the bottom of the aquifer followed by (c) groundwater extraction or vacuum pumping of soil gas to remove residual contamination. In some cases, groundwater extraction or vacuum pumping of soil gas is used exclusively. However, such methods may be inefficient since the conditions required to mobilize the organic fluid trapped in the pore spaces in the soil cannot be met and the dissolution or vaporization of the contaminant fluid in the flowing groundwater or soil gas is limited by mass transfer constraints imposed by the distribution of contaminants in regions not readily contacted by the flowing fluids. Therefore, in decontamination by water or gas pumping, it is not known how much water will have to be pumped, for how long, or whether the pumping will be effective.
Most of the substances comprising spills are only slightly soluble in water. (i.e., much less than 1 gram per hundred grams) and many are heavier than water. Fluid viscosities are generally less than water and boiling points are usually comparable to that of water. Once liquid contaminant is released into the soil in a sufficient quantity, it tends to sink toward the water table regardless of density. In the process of migration, this liquid follows zones of the highest permeability or fractures leaving behind small masses or ganglia trapped in some of the pores of the soil. The amount of organic liquid left behind in the pores is the residual saturation. Expressed as the fraction of the void space occupied by the hydrocarbon contaminate, residual saturations in saturated porous media have been reported as 10% for a light oil and 20% for a heavy oil; 0.75-1.25% (oil contamination) in a highly permeable media, and 7.5-12.5% within an unsaturated zone in a low permeability medium.
Liquids heavier than water move downward into the capillary fringe in the water table while liquids lighter than water tend to spread laterally when they encounter the capillary fringe and the water table. Groundwater withdrawal as a method of clean-up is intended to depress the water table in order to recover the spill (such as gasoline) as a separate phase. However, the amount of free product typically recovered using this method has been significantly less than the estimated amount of the spill. This can be partly explained by the complexity of the geometric distribution of the contaminated zone in a field situation. Because of natural depositional processes, even very uniform sedimentary deposits contain interfingering sequences of finer and coarser layers. This layering leads to lateral spreading of second-phase (nonaqueous phase) liquid during its downward migration and especially when it encounters a barrier such as the water table or a very low permeability layer. In addition, the water table is a dynamic surface which moves up and down in response to seasonal recharge and discharge along with local pumping. As a result, the zone contaminated by a lighter-than-water liquid may extend over the entire range of the height of water table fluctuations. This can result in lenses of the contamination phase, not only above the water table, but sometimes even below the water table.
The net effect of redistribution of the contamination phase by a dynamic water table is that a significant portion of the contamination phase may be effectively trapped in the pore spaces, and adsorbed on the soil grains, especially during the periods of lower water table levels. During period of high water tables, some of the contamination phase in large continuous lenses can be remobilized and is recoverable, however, much of it cannot be mobilized by the hydrodynamic forces produced in a typical pumping operation.
Methods have been utilized to attempt to remove trapped residual liquid (i.e., trapped either in liquid lenses or trapped in the pore spaces of the soil) by pumping water to remove the constituents dissolved in the groundwater and by vacuum pumping above the water table to remove volatile constituents. In both instances the rates of removal are slow since they are controlled by diffusion and solution kinetics.
Biodegradation may be effective in some situations, but it is also a relatively slow process, particularly when dealing with a separate liquid phase contamination.
The effectiveness of biodegradation is also highly dependent on soil conditions, which are difficult to control, such as oxygen levels, moisture content, temperature, availability of nitrogen and phosphorous, and pH.
U.S. Pat. No. 4,761,225 discloses removal of liquid hydrocarbons from the groundwater by using perforated well casings, a set of pump chambers and a control system which is powered by compressed air. Liquid hydrocarbons and groundwater are drawn up through the recovery unit into pump canisters by suction and then pressurized gas is directed to individual wells to push the liquid hydrocarbons from the wells to the storage recovery tank at ground level.
U.S. Pat. No. 4,832,122 discloses a method for decontamination by using injectors located below the water table to inject fluid or gases upwardly through the contaminated area. However, a disadvantage of injecting a fluid or gas upwardly through a contaminated zone is that the withdrawal of water through the extractor (usually horizontally located above the contaminated zone), is less efficient since the extraction is working against the force of gravity. Thus the extraction of water and other condensate is not optimally efficient.
An alternate way to remove a contaminant is to physically displace the contamination phase from the porous medium by the injection of steam. The present invention is directed to a method which improves upon the method of injecting steam into the contaminated zone as a means of recovering contaminants.
Accordingly, it is an object of the present invention to remediate and decontaminate a site contaminated with a waste material in an effective, rapid and safe manner.
Another object of the present invention is to provide a method of soil decontamination which volatilizes and displaces compounds which are not significantly volatile at ambient temperatures and pressures.
Another object of the present invention is to provide an improved technique of soil remediation which does not require excavation of the contaminated area.
Another object of the present invention is to provide an improved technique of soil remediation which minimizes the volume of contaminated water which must be removed from the contaminated site.
Further advantages of the invention will become apparent from the following description and from the practice of the invention.